Table 3 Standardised incidence ratios and 95% CI of Crohn's disease among 709 353 patients undergoing appendectomy in Swedish (1964 to 2004) and Danish (1977 to 2004) hospitals stratified by cause of appendectomy.

Cause of appendectomy	Person‐years	Obs	Exp	SIR (95% CI)	*p Value
Appendicitis	7 188 320	924	707.6	1.31 (1.22 to 1.39)	<0.001
With perforation	1 234 469	216	114.2	1.89 (1.66 to 2.16)
Without perforation	5 953 851	708	593.5	1.19 (1.11 to 1.28)
Mesenteric lymphadenitis	645 698	111	68.0	1.63 (1.36 to 1.97)
Other disease (non‐inflamed appendix)	3 244 837	620	314.1	1.97 (1.82 to 2.14)
Appendiceal disease except appendicitis	43 437	8	4.2	1.92 (0.96 to 3.83)
Non‐specific abdominal pain	654 377	138	69.6	1.98 (1.68 to 2.34)
Other and unspecified disease	2 547 023	474	240.4	1.97 (1.80 to 2.16)

Exp, Expected number of patients with Crohn's disease; Obs, observed number of patients with Crohn's disease; SIR, standardised incidence ratio.

*Test of homogeneity. P value < 0.001 whether using a three‐way (appendicitis, mesenteric lymphadenitis, other disease) or a six‐way (appendicitis with perforation, appendicitis without perforation, mesenteric lymphadenitis, appendiceal disease except appendicitis, non‐specific abdominal pain, other and unspecified disease) categorisation of the cause of appendectomy.